1. Technical Field
This application relates to a loudspeaker and, in particular, to a loudspeaker comprising a frame, a movable diaphragm that oscillates around a position of rest, and a suspension for mounting the diaphragm to the frame, in which the suspension comprises a flexible surround portion defining an enclosed space where at least a portion of the flexible surround portion is air-permeable.
2. Related Art
Loudspeakers may be constructed of a diaphragm, a moving voice coil secured to the diaphragm in a central area, a magnet motor gap system into which the moving voice coil is inserted with the coil being centered in the gap of the magnet motor system, a frame supporting the magnet system and enclosing the diaphragm on one half side of the diaphragm, and a suspension system.
In a cone loudspeaker, the guiding of the movable diaphragm may be achieved by a double mechanical guiding or suspension system. The suspension system may be comprised of two elements, a flexible deformable surround (front or outer suspension) that is secured to the frame in an edge area of the diaphragm and the spider (rear suspension) that guides the oscillation movement of the moving voice coil and of the diaphragm mounted to the coil. The excursion of the moving system may be limited by the maximum mechanical deformation of the spider. The stiffness of the suspension system is the sum of the stiffness of the flexible surround portion and of the stiffness of the spider.
Surrounds can be constructed from several materials including rubber, compressed foam rubber, corrugated cloth, paper, plastic, and the like. Often the word “roll” is used in place of “surround” when describing the front suspension. Roll surrounds have a single, large, semi-circular corrugation typically constructed from rubber, compressed foam rubber or treated fabric. Surrounds help keep the diaphragm centered and provide a portion of the restoring force that keeps the voice coil in the motor magnet gap. In addition to controlling the linear motion of the cone, the surround also acts as a major centering force for the loudspeaker's voice coil. This centering force prevents the voice coil and former from rocking and rubbing against the pole piece or top plate. The surround also provides a damped termination for the edge of the cone. The choice of thickness and material type for surround construction can alter the response of the loudspeaker.
The spider, that may be constructed from treated corrugated fabric, also keeps the voice coil centered, as well as providing a portion of the restoring force that maintains the voice coil within the gap. The stiffness of the spider can affect the loudspeaker's resonance. The spider also provides a barrier for keeping foreign particles away from the gap area.
Surrounds may be one of the limiting factors in designing long-excursion loudspeakers. Excursion is defined as the amount of linear length the diaphragm body can travel. With small roll diameters, the excursion may be limited by the surround's physical limits. Larger surrounds may have an attendant loss in effective diaphragm area for a loudspeaker of given outside diameter, thus, creating an inevitable trade-off. Excursion and cone area are two factors which contribute to a loudspeaker's volume displacement. The higher the volume displacement capability of a loudspeaker, the greater the loudspeaker's ultimate low frequency output potential can be.
Furthermore, loudspeakers may be divided into several categories. Loudspeakers that are designed to produce low frequencies are referred to as “woofers.” In these loudspeakers, the diaphragm is large and has a large range of excursion. Loudspeakers that are designed to produce high frequencies are referred to as “tweeters.” Tweeters may comprise smaller diaphragms that oscillate at a smaller range of excursion. Loudspeakers that are designed for medium frequencies are referred to as “mediums.” For these different kinds of loudspeakers different magnet systems and different guiding systems have been used.
Woofers, for example, may comprise a conically shaped diaphragm and a double mechanical guiding system including the surround portion and a spider. This double mechanical guiding system is used to properly guide the oscillating voice coil even at maximum excursions. Tweeters, in which the excursions of the moving diaphragm and the associated voice coil may be smaller, the use of spiders for the guiding system may not be necessary and the guiding system may consist only of a flexible surround portion.
Suspension systems may suffer from several drawbacks. For example, the linearity of the spider may not be very good due to parasite hysteresis effects and as the mechanical properties of the spider fatigue during use. In addition, the linearity of the flexible surround portion for fixing the diaphragm to the frame may not be very good as the geometry of the suspension may not be symmetric. In this instance, the oscillation around the position of rest may produce different restoring forces in an anterior and posterior direction.
Thus, to obtain a loudspeaker having a good sound quality, it is desirable that the suspension system be linear; in other words, that the restoring force be directly proportional to the excursion. Furthermore, it is desirable that the suspension system be symmetric, i.e., an excursion in the posterior and anterior direction of the loudspeaker should have the same restoring forces. However, the mechanical behavior of the suspension system at maximum excursions may be difficult to control because a good compromise between the linearity of the spider and the restoring force at maximum excursion is difficult to obtain. The loudspeaker may operate in a closed or vented box. At larger excursions of the moving system, the internal pressure in the box may modify the geometry of the flexible surround portion and may create acoustical distortions.
Symmetrical suspension systems have been attempted that include a diaphragm that is supported at its marginal edge on the frame by means of a surround that consists of an angular, hollow member of rubber-like flexible material that is substantially circular shape in cross-section. This surround is connected at its inside periphery to the marginal edge of the diaphragm and at its outside periphery to the frame. This hollow surround has several drawbacks. First, the surround forms a closed space in which air is contained. In circumstances in which the loudspeakers are exposed to the sun or subjected to heat, for example in a vehicle, the expansion of the heated air inside the closed surround may damage the surround. As the air volume within the surround depends on the temperature of the air inside the surround, the sound quality of the loudspeaker depends on the ambient temperature of the loudspeaker. In addition, this system is not linear. At high excursions of the diaphragm, the air in the closed box is much more compressed, resulting in higher restoring forces at maximum excursions of the voice coil.
Therefore, there is a need to provide a loudspeaker having a substantially linear and symmetrical suspension system, which improves the sound quality of the loudspeaker.